1. Field of the Invention
The invention relates to a hydraulic power steering system.
2. Discussion of Background
Japanese Patent Application Publication No. 2007-253829 (JP 2007-253829 A) describes a hydraulic power steering system that assists a steering operation by supplying hydraulic fluid from a hydraulic pump to a power cylinder, which is coupled to a steering mechanism of a vehicle, via a hydraulic control valve. In such a hydraulic power steering system, the hydraulic control valve is mechanically coupled to a steering member, such as a steering wheel, via a steering shaft, and the opening degree of the hydraulic control valve is adjusted in response to an operation of the steering member.
Japanese Patent Application Publication No. 9-263256 (JP 9-263256 A) describes a hydraulic power steering system in which a hydraulic control valve is not mechanically coupled to a steering member and the opening degree of the hydraulic control valve is controlled by an electric motor (valve driving motor). In such a hydraulic power steering system, an opening degree command value for the hydraulic control valve is computed on the basis of, for example, a steering torque and a vehicle speed, and drive control over the valve driving motor is executed on the basis of the opening degree command value. Note that an electric motor (pump driving motor) for driving a hydraulic pump is controlled on the basis of a pump rotation speed command value computed from a steering angular velocity (rotation speed feedback control).
In the hydraulic power steering system in which the opening degree of the hydraulic control valve is controlled by the valve driving motor, the steering member is not mechanically coupled to the hydraulic control valve. Therefore, the following problem may occur. That is, when the pressure of hydraulic fluid is decreased due to an abrupt change in the opening degree of the hydraulic control valve, a load of the hydraulic pump decreases and, as a result, the hydraulic pump attempts to increase its rotation speed. However, because the pump driving motor is subjected to rotation speed feedback control, the rotation speed of the hydraulic pump does not increase. Consequently, the amount of hydraulic fluid that is supplied to a power cylinder decreases.